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Cold case: The unsolved mystery of what lit Kepler’s supernova

Astrophile is Joshua Sokol's monthly column on curious cosmic objects, from the solar system to the far reaches of the multiverse

Supernova remnants as discovered by Johannes Kepler in 1604

X-ray: NASA/CXC/NCSU/M.Burkey et al; Optical: DSS

By Joshua Sokol

The facts of the case are as follows. On the night of 9 October 1604, Europe turned to the south-western sky, where Jupiter, Saturn and Mars were slated to assemble in Sagittarius. Some believed it would herald a radical transformation of the world.

The conjunction went as scheduled. But something else stole the spotlight in the nearby constellation Ophiuchus. A new star appeared, growing brighter and brighter for 20 days straight, more luminous than any planet, lingering and fading over the next year. It was the last great Milky Way supernova recorded by observers with their naked eye.

“We can be sure of only one thing,” wrote Johannes Kepler, who compiled detailed records of the event. “Either the star signifies nothing at all for mankind or it signifies something of such exalted importance that it is beyond the grasp and understanding of any man.”

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Supernovae of exalted importance

The still-expanding stellar shrapnel is called Kepler’s Supernova Remnant. Studying it is a bit like cosmic blood-splatter analysis. In hindsight, astronomers classify what happened back in 1604 as a Type Ia supernova: the kind that modern cosmology uses as a measuring stick to gauge the size and history of the universe.

Despite how much we depend on them, what causes Type Ia supernovae in general is unclear. In one scenario, mass from a neighbouring red giant star falls on a dense, hot white dwarf core, which then obliterates itself in a thermonuclear explosion. The red giant star that lit the fuse should survive the blast.

The other scenario is that Type Ia supernovae occur when two white dwarfs merge, wiping each other out.

Kepler’s supernova was a major historical event that happened unusually close to us. Astronomers have been checking out its aftermath for 400 years. You might expect that at least we would have a handle this Type Ia supernova. Far from it.

An unsolved case

The scene does contain what many think is a key clue. “Kepler’s supernova remnant is special,” says Jacco Vink at the University of Amsterdam, the Netherlands, who wrote a recent review paper about the mystery.

“There’s a lot of material in the supernova remnant that can only come from another star,” he says. The gas ejected from the supernova seems to be ploughing into other gas that was ejected earlier from the system. That hints at the involvement of a red giant star, which would have been expelling some of its atmosphere to space – not two white dwarfs.

But recent searches for that second star have drawn a blank. That could mean that there was a second star next to a white dwarf, but that the second star transformed into a white dwarf, too, shortly before the pair mutually annihilated each other.

It could also be that the second star is still hiding there, but disguised or disfigured by the blast – and is now brighter, fainter, or otherwise unrecognisable.

Astronomers are holding out hope that deeper searches for that second star could still find it, or that spectral studies of the remnant might provide new evidence dating back to the time of the explosion. Until then, a cold case smolders white-hot in the south-western sky.